A. Field of the Invention
The present invention relates to ultrasonic bonding machines of the type used to bond interconnecting wires to miniature electronic devices such as integrated circuits and magnetic read/write heads used in disk drive memories. More particularly, the invention relates to a method and apparatus for detecting absence of a bonding wire within the bore of an ultrasonic bonding tool, because of inadvertent breakage or miss-feeding of a bonding wire being used to make interconnections between bond sites.
2. Description of Background Art
Miniature electronic circuits, or xe2x80x9cmicro-circuits,xe2x80x9d are used in vast quantities, in a wide variety of consumer, commercial, industrial and military devices and equipment. The majority of such micro-circuits are of a type referred to as integrated circuits. Integrated circuits contain a number of active circuit elements such as transistors, and passive elements such as resistors and capacitors mounted on a substrate. Semiconductor integrated circuits consist of a small monolithic chip made of a semiconducting material such as silicon having discrete areas into which impurities are diffused to form circuit elements and having conductive paths between circuit elements on the chip formed by selectively etching metallized layers of the chip. In hybrid micro-circuits, circuit elements mounted on a ceramic substrate are usually interconnected by conductive ink paths on the substrate.
Functional portions of integrated circuits are typically in the form of very small, rectangular-shaped chips, ranging in size from 0.025 inch to 0.200 inch or more on a side. Input connections to integrated circuit chips are often made by bonding a very fine wire to conductive pads on the chips, the other end of each wire being bonded to a conductive terminal that is sufficiently large and robust to be soldered to conductors on a circuit board. Wire bonding of this type utilizes ultrasonic energy and/or heat to form an intermetallic bond or weld between the wire and metallic bond site. Such wire bonds are used to form interconnections between pads of an integrated circuit chip and terminals of a package used to enclose and protect the chip, and are also used to connect lead-out terminals to delicate read/write heads used in disk memories.
Bonding wires used to interconnect the pads of a semiconductor chip to terminals of a package containing the chip are generally made of aluminum or gold, and have a diameter of about 1 mil (0.001 inch) or less. Each interconnecting wire must be bonded to the upper surface of a small, typically rectangular-shaped, integrated circuit pad a few mils wide at one end of the wire to form a first bond site, and to a similarly shaped, larger package terminal comprising a second bond site.
The most common method of interconnecting wires between semiconductor chip pads and external terminals uses ultrasonic energy to form a welded bond at each end of a conducting wire. To form such bonds, the free end of a length of bonding wire is placed in contact with a pad. Then the tip of an ultrasonic transducer is pressed against the wire, and energized with ultrasonic energy for a short time interval. The combination of a normally downwardly directed pressure applied by the tool to the contact region between the lower surface of the wire and the upper surface of the pad, combined with a rapid lateral scrubbing action of the tool tip in a horizontal direction parallel to the pad, causes an inter-molecular diffusion bond, sometimes referred to as a xe2x80x9cweld,xe2x80x9d to be formed between the wire and pad. The free end of the bonded wire is then moved to another pad or terminal, and bonded thereto by the same process. After the last bond in a series of bonds has been thus formed, the wire is severed at the last bond site.
In view of the very small sizes of both the micro-circuit pads and bonding wire, it can be appreciated that ultrasonic bonding of connecting wires to integrated circuit pads must be performed using a tool mounted in a bonding machine that permits the tool to be manipulated to precisely controllable positions within a work area containing a workpiece.
Typical wire bonding machines used for ultrasonic welding of wires to micro-circuit pads include an elongated, generally cylindrically shaped force-applying member or xe2x80x9ctool.xe2x80x9d The tool is usually vertically disposed, and has a shank fastened at the upper end thereof to a source of ultrasonic energy, such as a piezoelectric transducer which is connected to an electrical energy source alternating at an ultrasonic frequency. Usually, the tool is connected to the transducer through a tapered horn structure that matches the acoustic input impedance of the tool to the output impedance of the transducer, which typically has a larger diameter than the tool shank.
One type of ultrasonic bonding tool used to bond wires to micro-circuit pads is referred to as a wedge bonder and has a flat lower working face adapted to press a bonding wire into contact with a pad, while ultrasonic energy is applied through the tool to the wire to form an ultrasonic weld. This working face is usually quite small, typically having a rectangular shape only about a few mils on a side, to permit bonding wire to small micro-circuit pads, without contacting adjacent circuit elements. The bonding process typically includes the steps of first viewing a particular workpiece pad and tool tip in a stereo microscope and video camera to align a workpiece relative to a bonding machine, and then using an automatic actuator system to position the tool tip at consecutive bond site locations on the workpiece, using a control system which employs pattern recognition logic.
In most wire bonding machines, the bonding tool is so constructed as to facilitate the positioning of bonding wire over a pad, prior to performing the bonding operation. Such bonding tools may include an upwardly angled lower face rearward of the working face, and a generally vertically disposed rear face. An angled bore or wire guide hole having an entrance aperture in the rear face and an exit aperture in the angled lower face of the tool enables bonding wire supplied from a reel mounted upwardly and rearwardly of the tool to be paid out through the exit aperture in the angled lower face of the tool. Typically, a remotely actuable clamp located rearward of the wire guide hole entrance and movable with respect to the tool is used to feed bonding wire through the guide hole.
The clamp used to push wire through the guide hole of a bonding tool usually consists of a pair of jaws or clamp blades that may alternately be closed to grip the wire, and opened to allow free travel of the wire. Generally, such clamps may be moved toward and away from the guide hole entrance, typically on a line of movement which coincides with the axis of the guide hole. To feed wire through the guide hole, the jaws of the clamp are first opened, and the clamp then moved away from the guide hole. The jaws are then closed to grip the wire, and then moved towards the guide hole, thus feeding wire through the guide hole.
In wire bonding machines of the type just described, the machine is used to translate the bonding tool to the proper position to bond wire to a first bond site of a pair of bond sites, such as a pad on an integrated circuit die, feed wire out through the guide hole exit aperture, move the tool to a second bond site and form a second bond. In this manner, any desired number of pads or other elements of a circuit can be connected together, in a procedure referred to as xe2x80x9cstitchxe2x80x9d bonding. After the second or last bond in a series of bonds has been made, the wire must be severed, to permit making bonds between other pairs of bond sites. In one method of severing the wire, the clamp blades are then closed upon wire rearward of the bonding tool, and the clamp is translated rearwards from a second bond site to exert tension on the bonding wire sufficient to sever the wire. The clamp is then fed forward to feed a new length of wire from the tool. Alternatively, the wire may be severed by a xe2x80x9ctable tearxe2x80x9d method, in which a table or platform holding a workpiece is translated forward from the tool to tension and thereby sever the wire, while the wire is held by closed clamp blades.
In moving a wedge bonding tool from a first bond site to a second bond site, the tool must be translated rearward from the first site to the second site, in a vertical plane containing both the longitudinal axis and wire-guide bore axis of the tool. This requirement results from the fact that wire paying out forwardly through the exit aperture of the bonding tool tip must remain in the plane containing the longitudinal and guide hole axes of the tool, to ensure that the wire will not bind on the exit aperture chamfer, or become twisted.
Because of the requirement for translating a wedge bonding tool from a first to subsequent bond sites in the plane of the bonding tool longitudinal axis and wire guide bore axis, existing wedge bonding methods require that a workpiece be rotated to align a direction vector between the two sites with the bonding tool plane, and subsequent translation of the bonding tool rearwardly in that plane along the direction vector.
One method of performing the required relative translations and rotations of a wedge bonding tool relative to a workpiece utilizes a support platform for the workpiece, which is translatable in an X-Y plane perpendicular to the longitudinal axis of the bonding tool, and rotatable in the Y-Y plane. With this method, the bonding tool need only be translatable downwardly, in a minus xe2x88x92Z direction to effect a bond, and upwardly in a plus+Z direction after a bond has been made.
The manufacture of production quantities of microcircuits generally requires the use of automated ultrasonic bonding machines performing bonding operations of the type described above, to achieve satisfactory production rates at reasonable unit costs per circuit. An example of such automated ultrasonic bonding machines is disclosed in the present inventor""s application Ser. No. 09/570,196, filed May 12, 2000, for an Automatic Ultrasonic Bonding Machine With Vertically Tiered Orthogonally Translatable Tool Support Plafforms. The disclosed machine includes a positioning mechanism for automatically translating the tip of an ultrasonic bonding tool by drive motors to precisely pre-determinable positions within a three-dimensional coordinate space containing a workpiece. The machine described in that application also provides means for translating a bonding tool in X-Y directions parallel to a plane containing a workpiece to position the tool tip over a particular bond site, translating the tool downwardly in a minus xe2x88x92Z direction to make an ultrasonic wire bond, translating the tool upwardly to withdraw the tool tip from the first bond site, and translating the tool in X-Y direction to position the tip over a subsequent intended bond site, and form thereat a subsequent bond. Thus, the disclosed machine eliminates the a requirement for a rotatable X-Y table for supporting a workpiece, and provides a highly effective method for making bonds on workpieces located on a conveyor, for example.
Operation of automated ultrasonic bonding machines typically requires an operator to select a pre-programmed operation cycle for the machine and arrange for a quantity of workpieces to be automatically presented to the machine, on a conveyor, for example. Pattern recognition logic is then used to position alignment targets of a workpiece precisely with respect to an ultrasonic bonding tool, whereupon a sequence of automatic bonding operations joining individual interconnecting wires between a plurality of a first and second bond sites may be initiated. Each work piece may require bonding dozens or even hundreds of individual interconnecting wires between pairs of separate bond sites.
In ultrasonic bonding an interconnecting wire between a pair of first and second bond sites, a length of wire which enters an entrance opening of a bore through the tool tip from a supply reel and protrudes from an exit opening of the bore located rearwards of a front foot of the tool is pressed down between the foot and a first bonding site such as a conductive pad on an integrated circuit chip. Ultrasonic energy is then applied to the tool, causing the tip of the tool to vibrate fore and aft at an ultrasonic frequency. Scrubbing action of the lower surface of the wire against the pad causes the contacting surfaces between the pad and wire to be ground together. This scrubbing action in turn results in the plastic deformation of microscopic surface protrusions, pushing the peaks of the protrusions into valleys of a contacting surface, dispersing impurities from the surfaces and bringing nascent molecules of the wire and pad so close together that they intermingle and form a solid-state diffusion bond. Even though such bonds are formed at room temperature, they are sometimes referred to as welds. The bond or weld secures the wire to the pad mechanically and in electrically conductive contact with each other. The tool tip is moved upwards from the first bond site, and translated rearward from the first bond site to position the tool tip foot over a second bond site, such as a terminal pad. The bonding tool is again moved downwards to press the length of wire against the second conductive pad, and ultrasonic energy once again applied to the tool tip to effect a second bond.
After a pair of bonds has been made as described above, a continuous length of bonding wire interconnects the first and second bond sites, and trails rearward away from the second bond site under the tool. The trailing portion of wire must be severed by either of two methods, to finish the interconnection and prepare the bonding tool to make another pair of first and second bonds. According to one method of severing a wire at a second bond site, jaws of a wire clamp located behind the tool are closed to grip the wire between the tool and a wire supply reel, and the clamp moved rearward to exert tension on the wire sufficient to break the wire. According to another method of severing a wire at a second bond site, sometimes referred to as a xe2x80x9ctable tear,xe2x80x9d a clamp secures the length of wire rearward of the tool, and a platform or table to which a workpiece is secured is translated forward from the tool. In both methods, tension exerted on wire causes it to break at the rear area or heel of the second bond.
After a length of bonding wire from a supply reel has been severed at a second bond site by either of the two methods described above, the wire clamp rearward of the tool is moved forward to feed wire outwardly through the bonding tool bore a distance sufficient to underlie the front foot area of the bonding tool, thus positioning the wire correctly for initiating another first bond of a pair of bonds.
A problem which can occur using automatic ultrasonic bonding of the type described above is accidental breakage of a length of bonding wire rearward of the bonding tool. Such breakage can be caused by imperfections in the wire supply from the reel, causing the wire to be weakened, excessive tension being exerted on the wire by the supply reel, or other such causes. Such accidental breakage of wire is quite problematic, for the following reasons.
In automatic ultrasonic bonding, the ultrasonic bonding tool is brought forcibly down against a length of bonding wire, to scrub the lower surface of the wire against a bonding site and thereby form a bond. If bonding wire is absent from its proper location below the foot of the bonding tool, the foot will directly contact and scrub the bonding site surface. This direct scrubbing action of a bonding tool foot on an integrated circuit pad or terminal pad generally degrades the surface to the extent that adequate wire bonds may not subsequently be made to the pad. Thus, even if the absence of wire is detected and bonding wire re-threaded into the bonding tool, the workpiece must usually be scrapped. Therefore, by the time an operator discovers by physical inspection that bonding wire from a supply reel has broken rearward of the bonding tool bore, a large number of workpieces may have been ruined. Accordingly, it would be desirable to provide means for detecting inadvertent breakage of a bonding wire at the time of its occurrence, so that a production run and ultrasonic bonding machine may be shut down to permit re-threading of the broken bonding wire into the bonding tool bore.
A number of schemes have detecting broken or missing wires in ultrasonic bonding machines have been proposed. For example, Toner in U.S. Pat. No. 6,179,197, Jan. 30, 2001, Missing Wire Detector discloses a method and apparatus for monitoring the presence of a bonding wire in a wire bonding machine in which an AC signal, specifically, the A.C. ultrasonic drive signal for the ultrasonic transducer of an ultrasonic bonding tool is coupled to a terminal of a peizorestive transducer from an ultrasonic drive current generator through a first, upper, input current sensing resistor, the other terminal of the transducer being electrically coupled to the transducer horn, which is connected to ground return for the current generator through a second, ground path current sensing resistor. When a bonding tool wedge is in electrically conductive contact through bonding wire and a feed clamp to a ground reference, current through the ground path resistor is less than that through the upper input current sensing resistor by about 15% confirming that the bonding wire is present. If the current difference is less than 15%, a voltage comparator connected to the high ends of the input current and output sampling resistors generates a warning signal indicating that bonding wire is not present. The disclosed method requires that the ultrasonic transducer and wedge be permanently electrically isolated from ground, and that an ultrasonic voltage be applied to the workpiece.
Some microcircuits are so delicate that even minute voltages applied to the circuit during bonding will damage or destroy the circuit. Accordingly, certain limitations of prior-art missing wire detection schemes, such as the requirement to apply an electric potential to a workpiece, were motivating factors in the present invention.
An object of the present invention is to provide an apparatus for detecting the absence of a bonding wire within the bore of an ultrasonic bonding tool.
Another object of the invention is to provide an apparatus for detecting the presence of a bonding wire within the bore of an ultrasonic bonding tool which includes means for electrically detecting electrical continuity between a segment of bonding wire within the bore, and the bore wall.
Another object of the invention is to provide an apparatus for detecting the presence of an ultrasonic bonding wire within the bore of an ultrasonic bonding tool tip, which includes means for alternately electrically conductively connecting and disconnecting the tool to a source of bonding wire supplied to the tool, and means for electrically detecting electrical continuity between a segment of the bonding wire located within the bore, and the bore walls during a time period when the bonding wire source is electrically disconnected from the tool.
Another object of the invention is to provide an apparatus for detecting the presence of an ultrasonic bonding wire within the bore of an ultrasonic bonding tool tip that includes means for electrically detecting electrical continuity between a segment of bonding wire within the bore and the bore wall, and means for vibrating the tool to move it relative to the wire segment sufficiently for contact to be made between the wire segment and bore wall.
Another object of the invention is to provide a method for detecting the presence of a segment of bonding wire within the bore of an ultrasonic bonding tool tip comprising the steps of electrically conductively grounding an ultrasonic tool during formation of an ultrasonic bond, ungrounding the tool after formation of a bond, determining by electrical measurement means whether electrical continuity exists between a source of bonding wire for the tool, and the wall surface of a bore within the tip of the tool and a segment of wire within the bore continuous with wire from the source, and re-grounding the tool preparatory to making another ultrasonic bond.
Another object of the invention is to provide a method for detecting the presence of a segment of bonding wire within the bore of an ultrasonic bonding tool tip comprising the steps of electrically conductively grounding an ultrasonic tool during formation of an ultrasonic bond, ungrounding the tool after formation of a bond, vibrating the tool tip at an amplitude sufficient to cause the wall surface of the bore to at least intermittently contact a wire segment within the bore, determining by electrical measurement means whether electrical continuity exists between a source of bonding wire for the tool, and the wall surface of a bore within the tip of the tool and a segment of wire within the bore continuous with wire from the source, and re-grounding the tool preparatory to making another ultrasonic bond.
Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims.
It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, I do not intend that the scope of my exclusive rights and privileges in the invention be limited to details of the embodiments described. I do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims.
Briefly stated, the present invention comprehends a method and apparatus for detecting the absence of a bonding wire within the bore of an ultrasonic bonding tool tip of an ultrasonic bonding machine and providing an alarm signal in that absence to thereby alert an operator that the bonding wire must be re-threaded into the tool to enable wire bonds to be made, and/or providing a halt signal to halt operation of the bonding machine until re-threading has been effected.
An apparatus for detecting broken or missing bonding wire according to the invention includes a switching and detection electronics module having a first terminal connected to a bonding wire supplied from a reel or other such supply source, and a second terminal connected to the ultrasonic bonding tool. In one embodiment the wire and supply reel are connected to ground, while the tool is electrically isolated from the chassis and other components of the bonding machine. The electronics module includes an electronically controlled switch, preferably a field effect transistor (FET), which, in a quiescent state of the module, conductively couples the bonding tool to chassis ground through the low impedance of the FET in its on-state. The electronics module also includes a FET driver circuit which turns off the FET switch upon receiving an external enable signal, disconnecting the tool from ground. Also included in the electronics module is a time delay circuit which produces a time-delayed enable signal upon receipt of the external enable signal, the time delay enable being coupled to one input of a two input NAND gate. The other input of the NAND gate is connected to the output terminal of an inverting ground detection buffer amplifier, an input terminal of which amplifier is connected to the bonding tool and a positive voltage source through a pull-up resistor. The output terminal of the NAND gate is connected to a module output terminal designated WIRE O.K.
When wire from a supply reel connected to chassis ground is in electrically conductive contact with the wall of the bonding tool bore, a low voltage level at the input terminal of the ground detection circuit produces a logic TRUE level at the output terminal of the detection circuit, which when NANDED with the delayed enable signal produces a LOW, TRUE signal at the output of the NAND gate, the LOW signal level being interpreted as signifying presence of a bonding wire contacting the tool. If wire is absent from the tool bore, the input terminal of the detector circuit is raised to a logic TRUE level by the pull-up resistor connected to a positive voltage, causing the output of the detector circuit to go FALSE, which in turn causes the output of the NAND gate and WIRE O.K. terminal to go to a high, FALSE state, indicating a broken or absent wire and triggering an alarm and/or halting automatic operation of the machine. The time-delayed ENABLE ensures that the FET switch has disconnected the tool from chassis ground prior to the time that the tool voltage is monitored to determine if the tool voltage has indeed been lowered by contact with a grounded bond wire, rather than by the FET switch still being in its ON state.
A method of detecting a broken or missing wire within the bore of an ultrasonic bonding tool according to the present invention includes providing from an external source such as a computer under software control an external ENABLE signal to the electronics module to momentarily electrically isolate the tool from ground for an input ENABLE period prior to forming a first bond, and determining by electrical measurement means whether the tool has been brought to a low, near-ground logic level at least momentarily during a time-delayed ENABLE period, by electrical contact made between a wire within the bore of the tool and the bore walls, a positive continuity test result producing a WIRE O.K. signal.
In an alternate embodiment of the apparatus and method, the ultrasonic bonding tool is permanently grounded, and the wire and supply reel are alternately electrically isolated from and connected to ground by the FET switch.
In a preferred embodiment of the method used with bonding tool wedges having a relatively shallow wire feed angle with respect to a workpiece, the tool is vibrated, preferably by energizing its ultrasonic drive transducer during the enable period, thus assuring that a wire within the bore of the tool will at least momentarily contact the tool bore wall and thus signify its presence therein. Upon completion of the enable period, the tool is once again grounded, and a bonding cycle initiated provided that a WIRE O.K. signal is detected.
In an embodiment of the invention suitable for use with deep access bonding tools which have bores which make relatively large angles to a workpiece, a wire feed bore disposed longitudinally through the shank of the tool and which bonding wire passes through before entering the rear entrance to the steeply angled tool tip bore has installed therein a thin insulating tube, to ensure that the wire comes into electrical contact within the tool only when within its angled tip bore.